Microlithography is used for producing microstructured components, for example integrated circuits. The microlithographic process is carried out with a lithography apparatus, which has an illumination system and a projection system. The image of a mask (reticle) illuminated via the illumination system is in this case projected via the projection system onto a substrate (for example a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection system, in order to transfer the mask structure to the light-sensitive coating of the substrate.
Driven by the desire for ever smaller structures in the production of integrated circuits, currently under development are EUV lithography apparatuses that use light with a wavelength in the range of 0.1 nm to 30 nm, in particular 13.5 nm. In the case of such EUV lithography apparatuses, because of the high absorption of light of this wavelength by most materials, reflective optical units, that is to say mirrors, have to be used instead of—as previously—refractive optical units, that is to say lens elements.
The positions of the mirrors which are arranged in a projection system for defining a beam path can be detected with the aid of sensors. In this case, the sensors are mounted for example on a mirror or on a sensor frame, wherein the position of the mirror relative to the sensor frame can be detected. A sensor frame can be divided into a plurality of oscillation-decoupled sensor subframes. By virtue of a plurality of sensor subframes being provided, the sensor subframes can have intrinsically smaller dimensions or a reduced mass. This reduces the quasi-static deformation as a result of low-frequency dynamic excitation at the system and thus an image oscillation. If a plurality of sensor subframes are present which are referenced to one another, a corresponding measurement complexity arises, which results in measurement inaccuracies. Furthermore, alignment inaccuracies between the sensor subframes can occur.
WO 2013/178277 A1 discloses a projection system having two sensor subframes, to which in each case three optical elements are referenced with the aid of sensors. Furthermore, the sensor subframes have additional sensors in order to detect position data between the sensor subframes.